page_title: History of molecular theory
section_title: 19th century
hierarchical_section_title: History of molecular theory / 19th century
context_page_description: In chemistry, the history of molecular theory traces the origins of the concept or idea of the existence of strong chemical bonds between two or more atoms.
  The modern concept of molecules can be traced back towards pre-scientific and Greek philosophers such as Leucippus and Democritus who argued that all the universe is composed of atoms and voids. Circa 450 BC Empedocles imagined fundamental elements and "forces" of attraction and repulsion allowing the elements to interact. Prior to this, Heraclitus had claimed that fire or change was fundamental to our existence, created through the combination of opposite properties. In the Timaeus, Plato, following Pythagoras, considered mathematical entities such as number, point, line and triangle as the fundamental building blocks or elements of this ephemeral world, and considered the four elements of fire, air, water and earth as states of substances through which the true mathematical principles or elements would pass. A fifth element, the incorruptible quintessence aether, was considered to be the fundamental building block of the heavenly bodies.
context_section_description: Similar to these views, in 1803 John Dalton took the atomic weight of hydrogen, the lightest element, as unity, and determined, for example, that the ratio for nitrous anhydride was 2 to 3 which gives the formula N₂O₃. Dalton incorrectly imagined that atoms "hooked" together to form molecules. Later, in 1808, Dalton published his famous diagram of combined "atoms":
  Amedeo Avogadro created the word "molecule". His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to Partington's A Short History of Chemistry, that:
  The smallest particles of gases are not necessarily simple atoms, but are made up of a certain number of these atoms united by attraction to form a single molecule.
  Note that this quote is not a literal translation.  Avogadro uses the name "molecule" for both atoms and molecules.  Specifically, he uses the name "elementary molecule" when referring to atoms and to complicate the matter also speaks of "compound molecules" and "composite molecules".
  During his stay in Vercelli, Avogadro wrote a concise note (memoria) in which he declared the hypothesis of what we now call Avogadro's law: equal volumes of gases, at the same temperature and pressure, contain the same number of molecules.  This law implies that the relationship occurring between the weights of same volumes of different gases, at the same temperature and pressure, corresponds to the relationship between respective molecular weights. Hence, relative molecular masses could now be calculated from the masses of gas samples.
  Avogadro developed this hypothesis in order to reconcile Joseph Louis Gay-Lussac's 1808 law on volumes and combining gases with Dalton's 1803 atomic theory. The greatest difficulty Avogadro had to resolve was the huge confusion at that time regarding atoms and molecules—one of the most important contributions of Avogadro's work was clearly distinguishing one from the other, admitting that simple particles too could be composed of molecules, and that these are composed of atoms. Dalton, by contrast, did not consider this possibility.  Curiously, Avogadro considers only molecules containing even numbers of atoms; he does not say why odd numbers are left out.
  In 1826, building on the work of Avogadro, the French chemist Jean-Baptiste Dumas states:
  Gases in similar circumstances are composed of molecules or atoms placed at the same distance, which is the same as saying that they contain the same number in the same volume.
  In coordination with these concepts, in 1833 the French chemist Marc Antoine Auguste Gaudin presented a clear account of Avogadro's hypothesis, regarding atomic weights, by making use of "volume diagrams", which clearly show both semi-correct molecular geometries, such as a linear water molecule, and correct molecular formulas, such as H₂O:
  In two papers outlining his "theory of atomicity of the elements" (1857–58), Friedrich August Kekulé was the first to offer a theory of how every atom in an organic molecule was bonded to every other atom.  He proposed that carbon atoms were tetravalent, and could bond to themselves to form the carbon skeletons of organic molecules.
  In 1856, Scottish chemist Archibald Couper began research on the bromination of benzene at the laboratory of Charles Wurtz in Paris.  One month after Kekulé's second paper appeared, Couper's independent and largely identical theory of molecular structure was published.  He offered a very concrete idea of molecular structure, proposing that atoms joined to each other like modern-day Tinkertoys in specific three-dimensional structures. Couper was the first to use lines between atoms, in conjunction with the older method of using brackets, to represent bonds, and also postulated straight chains of atoms as the structures of some molecules, ring-shaped molecules of others, such as in tartaric acid and cyanuric acid.  In later publications, Couper's bonds were represented using straight dotted lines (although it is not known if this is the typesetter's preference) such as with alcoho
caption: Joseph Loschmidt's molecule drawings of ethylene H2C=CH2 and acetylene HC≡CH (1861)
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